Electrolyser and energy system

ABSTRACT

An electrolyzer operates within an energy system, for example to provide grid services, energy storage or fuel, or to produce hydrogen from electricity produced from renewable resources. The electrolyzer may be configured to operate at frequently or quickly varying rates of electricity consumption or to operate at a specified power consumption. In one process of operating an electrolyzer, a series of dispatches is received indicating a specified power consumption for a period of time. The dispatches may occur at least once every 30 minutes. The electrolyzer is operated according to the dispatches. Hydrogen produced by the electrolyzer is discharged to a natural gas system.

RELATED APPLICATION

This application claims the benefit under 35 USC 119 of U.S. ProvisionalApplication No. 61/652,263 filed May 28, 2012. U.S. ProvisionalApplication No. 61/652,263 is incorporated by reference.

FIELD

This specification relates to electrolysers and methods of operatingelectrolysers in an energy system, for example to produce hydrogen forenergy storage or fuel, or to provide electrical grid services.

BACKGROUND

European Patent EP 1 177 154 B1 is incorporated herein by this referenceand describes an energy distribution network for providing an amount ofhydrogen required from an electrolyser by a user. The network comprisesan electrical energy source, an electrolyser and a controller. Thecontroller receives and processes control inputs including datapertaining to a demand for hydrogen by a user. The controller isconnected to the electrolyser and controls the generation of hydrogen bythe electrolyzer based at least in part on the control inputs.

Canadian Patent CA 2 511 632 C is incorporated herein by this referenceand describes an energy network having a plurality of power stations anda plurality of loads interconnected by an electricity grid. The loadsmay include electrolysers. The network has a controller that isconnected to the stations and the loads. The controller is operable tovary the available power from the power stations or to adjust thedemands from the electrolysers to provide a desired match ofavailability with demand. Hydrogen may be produced as a transportationfuel with specific verifiable emission characteristics.

INTRODUCTION TO THE INVENTION

The following discussion is intended to introduce the reader to thedetailed description to follow, and not to limit or define any claimedinvention.

An electrolyser operates within an energy system, for example to providegrid services, energy storage or fuel, or to produce hydrogen fromelectricity produced from renewable resources. The electrolyser may beconfigured to operate at frequently or quickly varying rates ofelectricity consumption or to operate at a specified power consumption.

In some cases, a process has steps of providing an electrolyser andreceiving a series of dispatches indicating a specified powerconsumption for a period of time. The dispatches may occur at least onceevery 30 minutes. The electrolyser is operated according to thedispatches. Hydrogen produced by the electrolyser while operatingaccording to the dispatches is discharged to a natural gas system.

In some cases, an energy system has an electrical grid, an electrolyserand a natural gas system. The electrolyser is operated to provide a gridservice and discharges hydrogen into the natural gas system.

In some cases, a process has steps of providing an electrolyser,operating the electrolyser according to a dispatch order from anelectrical grid operator or according to a grid services contract anddischarging the hydrogen produced while so operating to a natural gaspipeline.

In some cases, a method of storing excess electrical energy in a gridhas steps of converting the excess energy into hydrogen and injectingthe hydrogen into a natural gas system.

In some cases, a method of making a virtual transfer of electricity hassteps of consuming electricity through an electrolyser operating at afirst location within an electrical grid to produce hydrogen, injectingthe hydrogen into a natural gas system at a first location, extractingnatural gas from the natural gas system at a second location, burningthe natural gas to produce electricity, and supplying the producedelectricity to a second location within the electrical grid.

In some cases, a method of marketing a virtual sale of hydrogen hassteps of consuming electricity through an electrolyser to producehydrogen, injecting the hydrogen into a natural gas system, measuringthe amount of hydrogen injected into the natural gas system, measuringan amount of natural gas withdrawn from the natural gas system by acustomer and invoicing the customer for an amount of hydrogenconsumption equivalent to at least a portion of the amount of naturalgas withdrawn. Optionally, the electricity may comprise electricityactually produced or deemed to have been produced from a renewableresource.

In some cases, an electrolyser has multiple stack assemblies each havinga separate power supply. Optionally, the electrolyser has a controlleradapted to operate the multiple stack assemblies at different rates ofpower consumption at the same time.

In some cases, an electrolyser has multiple stack assemblies that ventupwards to shared gas separators.

In some cases, an electrolyser has a controller and an electricitymeter. The controller is adapted to operate a DC power supply so as toconsume electricity at a pre-determined rate. Optionally, the controllermay operate the DC power supply so as to consume electricity at apre-determined rate when electricity is available at or below apredetermined price.

In some cases, a method of operating an electrical grid has steps ofimporting imbalance energy and consuming the imbalance energy through anelectrolyzer connected to a natural gas system.

In some cases, a method of operating an electrical grid has steps ofoperating a generator to produce electricity in excess of an amountrequired to operate a grid and consuming the excess electricity in anelectrolyser connected to a natural gas system. There may be a furtherstep of comparing the marginal cost of the excess electricity less anycost of not producing the excess electricity to a market value of thehydrogen.

In some cases, a process has steps of consuming electricity through anelectrolyser during a first period of time to produce hydrogen andinjecting the hydrogen into a natural gas system and, during a secondtime period, burning natural gas to produce electricity.

In some cases, a natural gas system has a customer gas meter comprisinga hydrogen concentration sensor. Data relating to flow rate and hydrogenconcentration is converted into an equivalent flow rate of natural gas.

In some cases, a method of operating a natural gas fuelling station ornatural gas fired electrical generating station has steps of measuringthe concentration of hydrogen in natural gas withdrawn from a pipelineand adding hydrogen to produce a gas mixture with a specified hydrogenconcentration.

In some cases, a method of operating a natural gas fuelling station ornatural gas fired electrical generating station has steps of enriching amixture of hydrogen and natural gas withdrawn from a pipeline and addingnatural gas to produce a gas mixture with a specified hydrogenconcentration.

In some cases, a method of operating an electrolyser has steps ofreceiving data related to a maximum amount of hydrogen that may beinjected into a gas pipeline and a) controlling the electrolyser toconsume no more than an amount of electricity that will produce themaximum amount of hydrogen, b) venting excess hydrogen or c) sending asignal to a grid operator indicating a corresponding maximum amount ofhydrogen that can be produced. The data may include the flow rate ofnatural gas in the pipeline.

The elements and steps described in the cases above may be used in thecombinations described in the cases, in a combination of any one of thecases described above with any element or step found in another case orin the detailed description to follow, or in other combinations.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic representation of an energy system comprising anelectrolyser.

FIG. 2 is a schematic representation of a portion of the energy systemof FIG. 1 including a connection between the electrolyser and a gaspipeline.

FIG. 3 is a schematic representation of various electrical components ofthe electrolyser of FIG. 1.

FIG. 4 is a schematic representation of hydraulic and gas circuits ofthe electrolyser of FIG. 1.

FIG. 5 is a schematic representation of an optional hydrogen enrichedgas withdrawal system for the energy system of FIG. 1.

DETAILED DESCRIPTION

Water electrolysis, called electrolysis in this specification, convertselectrical energy into chemical energy in the form of hydrogen. Thehydrogen is most valuable when consumed as an essentially pure fuel orindustrial chemical. However, the hydrogen also has value when blendedwith other gasses, the value tending to decline roughly with hydrogenconcentration. The hydrogen can also be converted back into electricity.Further, in some situations the ability of an electrolyser to consumepower can provide a valuable service, for example helping to balance orregulate an interconnected transmission system, also called anelectrical grid. Hydrogen can be considered as a fuel or industrialchemical, but can also be seen as an energy storage or transportingmedium. An electrolyser can be considered as a device for producinghydrogen, and also as a device for providing electrical grid services.

Grid services, which may also be called ancillary services or reserves,include various services that help maintain reliable operation of agrid. A grid system operator, sometimes call an independent systemoperator (ISO), electric system operator (ESO) or transmission systemoperator (TSO), may offer contracts for various kinds of grid services.Grid services include operations that assist an electrical grid operatorin managing a control area, or that can be used to either reduce orfacilitate energy transfers between control areas. Of particularinterest herein, some grid service contracts require the owner of avariable load to respond to dispatch orders made by a grid systemoperator for the purpose of balancing total production and consumptionon the grid, for example to avert or correct a short term imbalance.Electrical generators and controllable loads are both considered to beassets of the grid that can potentially provide grid services. The mostsignificant loads in a grid tend to be industrial processes. However,industrial processes tend to operate most efficiently at steady statesand so only some industrial processes may be used to provide gridservices, and the potential value of their grid services is typicallysmall.

In order to provide grid services, a process is required to operate atleast with a variable rate of power consumption. The potential value ofthe grid services that can be provided by a variable load are increaseda) if the rate, frequency, or size of the potential change in powerconsumption is increased, b) if the process is able to operateaccurately at a specified rate of power consumption or c) if the processcan be controlled by the grid operator.

The rate, frequency and size of a change in power consumption isrelevant to the value of grid services because very few loads or powergenerating assets are able to alter their consumption or productionquickly, frequently or by a large amount. Renewable energy generatingassets such as solar panels or wind turbines fluctuate more frequentlyand rapidly, and to a greater extent, than conventional generatingassets. Integrating high levels of renewable energy generating assets ina grid therefore requires a corresponding increase in the ability toalter other assets frequently and quickly, and to a correspondingdegree.

The ability to operate accurately at a specified rate of powerconsumption is valuable since a load that can follow dispatch ordersprecisely helps provide faster and simpler resolution of imbalances thana less precise asset. In market based systems, reduced costs may onlyapply to a specified consumption.

Enhanced control by the grid operator would increase the range ofproblems that can be solved by a controllable load. For example, powerto an ordinary interruptible load can be cut to prevent voltage in thegrid from declining below a minimum in an emergency but then the gridoperator does not have precise control over when the load will return.It would be of greater value to the grid operator if the duration of aload interruption was also controllable, and if the load could also beused to prevent an over-voltage in the grid. The value is higher if thegrid operator's dispatch is essentially mandatory or almost alwaysfollowed, rather than being subject to acceptance by the asset owner.Actual control, wherein the grid operator can send a control signal tothe load controller rather than a dispatch order to a load operator,also provides enhanced value. Variable control, wherein the gridoperator can dispatch an order or send a control signal specifying adesired rate of power consumption, provides more value than the mereability to completely shut down a load.

A process is useful if it can provide any one or more of the advantagesdescribed above, but many of them are difficult to provide. Regardingthe rate, frequency and magnitude of changes in power consumption, mostindustrial processes are constrained in their ability to respond to arequested change by one or more of a limited range of efficient processoperation, mechanical components used in the process, and the need toproduce a product. The mechanical components wear out or fail morefrequently when they are not operated in steady states. The rate ofproduct production may need to satisfy physical or market constraints.Regarding the ability to operate at a specified rate of powerconsumption, most industrial processes are controlled by specifying aproduction rate, not by specifying power consumption. Regarding controlby the grid operator, this interferes with the industrial manager'sability to optimize their process according to other constraints.

In order to increase its potential to provide grid services, anelectrolyser is preferably configured to have one or more of the abilityto operate at frequently, quickly or widely variable rates orelectricity consumption, to operate at a specified rate of powerconsumption, and to permit control by a grid operator.

One or more of these attributes can also be useful when not providingservices to a typical large operating authority controlled grid. Forexample, in order to produce hydrogen from renewable energy, anelectrolyser may be connected directly, or in a microgrid, to anelectrical generator. The generator may comprise, for example, a windturbine, a solar panel, a thermal solar device, or a generator burningbiogas. In such a direct or minimally buffered system, the need forelectrical energy storage can be reduced to the extent that theelectrolyser can operate with whatever amount of power is being producedin real time.

In another example, an electrolyser may be connected to an electricalgrid that allows for direct contracts between energy generators orloads, or that provides an auction or other market for dispatch ordersor service requests from the grid operator. In these situations, acontracted low electricity price may apply only to a specified time andamount of energy purchased. Any discrepancy between the contracted andactual time or amount of electricity consumed is likely to result in anincreased payment by the electrolyser operator. Since contracts mayspecify an electricity purchase at a rate varying in a step wise mannerover time, electricity costs are minimized if the electrolyser canfollow the specified steps accurately. This requires rapid changesbetween precisely specified consumption rates. Further, in systems whereenergy purchases can be verified or designated as coming from arenewable or low greenhouse gas generating source, claiming credits,offsets or other benefits may require a high level of correspondencebetween data relating to renewable energy production or purchase anddata documenting actual electricity used.

The presence of a strong market for essentially pure hydrogen, or atleast hydrogen at a higher concentration, may require balancingproduction restraints with the value of providing grid services. On theother hand, when the market for pure or high concentration hydrogen hasbeen satisfied, or is likely to be satisfied incidentally by theprovision of grid services, then providing grid services may be morevaluable that providing any specific amount of further hydrogenproduction. Connecting the electrolyser to a natural gas system providesa market, and a physical storage and transportation system, capable ofaccepting essentially any quantity of hydrogen that might be produced bythe electrolyser at essentially any time. The electrolyser can then becontrolled to provide grid services of the highest value essentiallywithout concern for the timing or amount of hydrogen production.

The further description to follow describes an energy system having anelectrical grid, an electrolyser and a natural gas system. Some hydrogenproduced by the electrolyser may be used for higher value markets, buthydrogen may also be injected into the natural gas system. The design ofthe electrolyser facilitates operating in a manner that provides gridservices. The system facilitates extracting value from the hydrogeninjected into the natural gas system.

FIG. 1 shows an energy system 10 having an electrolyser 12. Theelectrolyser 12 is connected to an electrical grid 14 within a controlarea 16. The electrical grid 14 is further connected to other electricalgrid control areas through an interchange transmission line 18. Withinthe control area 16, the electrical grid 14 is also made up of internaltransmission lines 20 of varying capacities. Portions of the interchangetransmission lines 18 within the control area 16 can also be consideredto be internal transmission lines 20.

The transmission lines 20 are connected to generators 22 and loads 24.The electrolyser 12 is also a load 24. A grid operator 26 is responsiblefor maintaining a balance of power production and consumption within thecontrol area 16 and, if necessary, for arranging for imports or exportsof electricity from or to other control areas through the interchangetransmission lines 18. The grid operator 26 may have communication tovarying degrees with one or more of the generators 22 and loads 24through one or more communications links 28. The grid operator hascontrol, to varying degrees, over at least some of the generators 22 andloads 24. Controlled generators 22 and loads 24 may be called assets.

The communications link 28 may, for example, allow the grid operator 26to convey a dispatch order and to receive a message indicatingacceptance, denial or modified acceptance of a dispatch order by thecontrolled asset 22, 24. Alternatively, the communications link 28 mayconvey an electronic signal directly to an asset 22, 24 in a machineusable form. Further alternatively, the communications link 28 may beindirect, for example in the nature of a market offering of a dispatchorder or service request, or an offer to provide electricity from aspecified generator 22 or the grid 14 generally at a specified cost,price or time. The asset 22, 24 may need to bid on, or contract for, theorder, request or electricity and may receive confirmation of itsobligation through an intermediary, such as a broker or automatedauction system.

Other generators 22 or loads 24 may operate essentially outside of thecontrol of the grid operator 26. Uncontrolled generators 22 or loads 24are typically connected to the electrical grid through a meter whichallows the grid operator 26 to at least know the production orconsumption of generators 22 and loads 24 connected though the meter.

Alternatively, the grid 14 may take other forms. For example, in aremote community the grid 14 may be contained within a control area 16having no interconnections with other grids 14 or control areas 16. In acaptive or privately owned electrical system, or part of an electricalsystem, an electrolyser 12 may be connected through a transmission line20 more nearly directly to a generator 22. In both of these cases, thefunctions of the grid operator 26 could be simplified to the point wherethese functions are essentially automated. The grid operator 26 may be aprogrammable logic controller, computer, or other programmable devicerather than an agency or company employing people and using programmabledevices.

In another alternative, a load aggregator may manage at least someaspects of the operation of multiple loads 24, including an electrolyser12, and provide grid services based on the combined abilities of themultiple loads. The aggregator may manage multiple electrolysers 12, ora mixture of one or more electrolysers 12 and one or more other loads24. In this case, although the load aggregator may be a private company,or other entity separate from the grid operator 26, the load aggregatorcan be considered to be part of the grid operator 26 and theelectrolyser operator.

The grid operator 26 balances the production and consumption ofelectricity in the control area 16 such that, among other things, thevoltage in the grid 14 is generally stable. In addition, the amount ofelectricity carried by each transmission line 18, 20 must be kept belowthe maximum capacity of each transmission line 18, 20. In FIG. 1, thegrid operator 26 is responsible for managing both the balance ofproduction and consumption and transmission constraints. Optionally, thetransmission constraints may be controlled by one or more otheroperators that coordinate with the grid operator 26. In this case, theseother operators may be considered to be part of the grid operator 26.The grid operator 26 may use the electrolyser 12 as a controllable loadto aid in balancing or regulating the grid or to manage transmissionconstraints. For example, an excess of electricity produced in one areaof the grid 14 can be consumed in the electrolyser 12 to prevent anover-voltage in the grid 14, to prevent electrical flow through atransmission line 18, 20 from exceeding its capacity, or both.

The grid operator 26 may also arrange transfers of electricity to orfrom other control areas 16 through an imbalance market or regulator.Grid operators 26 typically try to avoid such transfers. This is becausetimes when the grid operator 26 needs to import electricity tend to betimes when imbalance energy is costly. Times when the grid operator 26must export energy tend to be times when the price of electricity is lowor even negative.

The grid operator 26 can use the electrolyser 12 to absorb energy thatwould otherwise need to be exported or that can be purchased fromanother control area at a negative price. The grid operator 26 can alsokeep a generator 22 that is difficult or wasteful to throttle back inproduction to the extent that the electrolyser 12 can consume theelectricity produced. Such use of the electrolyser 12 may be by way of adispatch order by the grid operator 26, or by way of a market offering.

The energy system 10 includes a natural gas system 30. The electrolyzer12 is connected to the natural gas system 30 through a hydrogen outlet32. The natural gas system 30 comprises pipelines 34 of varyingcapacities which carry natural gas from a natural gas supply 42 to gasconsumers 40. Natural gas system 30 may also comprise one or morereservoirs 36 for storing natural gas outside of the pipelines 34.Optionally, the hydrogen outlet 32 may be connected to a hydrogenpipeline 38 that carries hydrogen to a gas consumer 40 without passingthrough the natural gas system 30. The natural gas system 30 is shown inFIG. 1 to be apart from the control area 16 to simplify the Figure. Thenatural gas system 30 and control area 16 are likely to overlap on theground. In particular, gas consumer 40 may also be either a load 24 or agenerator 22 in the control area 16.

Electrolyser 12 receives electricity from a transmission line 20 andproduces hydrogen, at least during some times. Some or all of thehydrogen may be injected from the hydrogen outlet 32 into a natural gaspipeline 34. In this way, electrolyser 12 stores energy in the energysystem 10 by converting electrical energy into hydrogen and storing thehydrogen in the natural gas system 30. In some cases, the hydrogen mayeventually be reconverted into electricity by being burned by a gasconsumer 40 that is a natural gas fired generator 22. Convertingelectricity to hydrogen through an electrolyser 12 in one part of thecontrol area 16 and generating corresponding electricity from a naturalgas fired generator 22 in another part of the control area 16 provides avirtual transfer of electricity through the natural gas system 30.

Regardless of whether or when the hydrogen actually reaches a naturalgas fired generator 22 or not, the hydrogen displaces natural gasconsumption and reduces the need to input natural gas into the naturalgas system 30. The hydrogen can be deemed in a management, billing,tracking, tax, carbon offset, carbon credit or other system or processto have been burned at any time by any gas consumer 40.

FIG. 2 shows further details of a portion of the energy system 10,particularly the connection between the electrolyser 12 and the naturalgas system 30. The electrolyser 12 shown comprises a core 50, interimstorage 52 and a compressor 54. Interim storage 52 and compressor 54 arepreferred but optional since some electrolysers 12 can be operated tooutput hydrogen at a pressure high enough to be injected into thenatural gas system 30. However, direct connection between the core 50and a natural gas pipeline 34 under conditions of variable powerconsumption is likely to cause a high persistent pressure or pressurefluctuations in the electrolyser stacks. Both conditions increase thelikelihood and magnitude of hydrogen leaks but can be avoided byproducing hydrogen at less than pipeline pressure and providing acompressor 54.

The division of components shown in FIG. 2 between the core 50, theremainder of the electrolyser 12, and the area outside of theelectrolyser 12 is arbitrary. For example, interim storage 52 and thecompressor 54 can be integrated into the balance of plant in the core50. Alternatively, the interim storage 52 and compressor 54 can belocated outside of a building housing the electrolyser 12 or even in aremote location.

In FIG. 2, the electrolyser core 50 produces hydrogen at less than thepressure in a natural gas pipeline 34 of the natural gas system 30. Forexample, the core 50 may produce hydrogen at about 100 psig. Interimstorage 52, if provided, is intended primarily to aggregate producedhydrogen so that compressor 54 does not need to be operated asfrequently or at a rate matching the rate of hydrogen production, and toprovide a buffer against creating suction in the core 50. Compressor 54may be, for example, a positive displacement pump. A storage pressuregauge 66, in fluid communication with the interim storage 52, monitorsthe pressure in the interim storage. Compressor 54 is turned on when thepressure gage 66 indicates that a high pressure set point has beenreached. Compressor 54 is turned off when the pressure gage 66 indicatesthat a low pressure set point has been reached. A recycle loop 70 with aregulator valve 68 may be added to prevent the compressor outletpressure from exceeding a predetermined maximum.

Hydrogen outlet 32 feeds into the gas pipeline 34 through an outputvalve 56 and a gas meter 58. The output valve 56 may be, for example, aback pressure regulator that opens only when the hydrogen pressureexceeds a minimum pressure selected to be above the likely maximumpressure in natural gas pipeline 34, for example 500 to 800 psig.Optionally, the output valve 56 may sense the actual pressure in thenatural gas pipeline 34 electronically or pneumatically and adjust theminimum pressure to follow the pressure in the natural gas pipeline 34,plus a safety factor. This avoids excess hydrogen compression whichconsumes energy. The gas meter 58 records the amount of hydrogen thatflows into the gas pipeline for one or more billing or management uses.Optionally, there may be two or more gas meters 58, one owned and usedby the operator of the electrolyser 12 and one owned and used by theoperator of the natural gas system 30. Alternatively, both of theseoperators may be one company, or they may share date from a single gasmeter 58. The gas meter 58 may be read manually from time to time butpreferably the gas meter 58 is capable of transmitting data, typicallyat regular polling intervals.

The electrolyser core 50 includes a water-hydrogen separator and a gasdrier. A humidity sensor in the electrolyser 12 checks that the hydrogenhas been reduced to gas pipeline standards before it leaves theelectrolyser 12. An oxygen sensor checks to make sure that the hydrogendoes not contain oxygen in excess of the electrolyser safety standardsor the gas pipeline standards. In addition to these internal sensors,the natural gas system operator may require its own oxygen and humiditysensors near the point of entry into the gas pipeline 34. Optionally,additives may be added to the hydrogen, for example anti-embrittlementadditives to improve the compatibility of the hydrogen with materials inthe natural gas system 30 or materials used by the gas consumer 40.

Electrolyser 12 has one or more controllers 60 that perform one or morecontrol processes. In one control process, the controller 60 determinesa maximum rate of hydrogen injection over time and prevents theelectrolyser 12 from injecting hydrogen at more than the maximum rate.The maximum rate may be determined by a preselected maximum hydrogenconcentration, for example 2% to 20% by volume, that applies to allcomponents of the natural gas system. In this case, the maximum hydrogenconcentration can not be exceeded in the natural gas pipeline 34 thatreceives the hydrogen. The rate of hydrogen injection at any timetherefore can not exceed a maximum percentage of the natural gas flowrate in that natural gas pipeline 34 at that time. To check for thiscondition, the controller 60 receives a signal A from a natural gaspipeline flow meter 62 connected to the pipeline 34 and a signal B fromthe gas meter 58. Assuming that there are no upstream electrolyser 12 orthe producer of hydrogen, the controller 60 compares the flow indicatedby signal B to the flow indicated by signal A multiplied by the maximumpercentage.

Various other methods of determining a maximum hydrogen injection rate,or whether a maximum allowable hydrogen concentration has or will beexceeded, can also be used. For example, if there is an upstreamhydrogen producer, the controller 60 can be connected to a hydrogensensor upstream of the hydrogen outlet 32 and further consider thehydrogen concentration in the gas pipeline 34 upstream of the hydrogenoutlet 32. Alternatively, the controller 60 may be connected to ahydrogen sensor downstream of the hydrogen outlet 32 to check directlywhether the hydrogen concentration exceeds the maximum hydrogenconcentration. In other alternatives, the controller 60 or anelectrolyser operator may respond to information obtained from an entitythat manages the natural gas system 30 indicating the natural gas flowrate in the gas pipeline 34 or indicating directly, or after furthercalculation, whether the natural gas system 30 can accept more hydrogen.In a case where information is obtained from an entity that manages thenatural gas system 30, that entity may be able to determine a maximumrate of injection that is not limited by a single maximum hydrogenconcentration that applies to all components of the natural gas system30. In this case, the maximum rate of injection may consider, forexample, dilution downstream of the gas pipeline 34 caused by areservoir 36 or incoming flow of natural gas. Optionally, a hydrogenpipeline 38 may transport hydrogen directly to a reservoir 36 in whichcase whether further hydrogen injection is permitted may be determinedby whether the hydrogen concentration in the reservoir 36 exceeds aselected maximum concentration.

The controller 60 compares the rate of hydrogen production, as indicatedby gas meter 58 or an internal gas meter, to the maximum rate ofinjection continuously or at polling intervals. As long as the rate ofhydrogen production remains below the maximum rate of injection, therate of production can be determined by other factors. However, if therate of hydrogen production exceeds the maximum rate of injection, thenthe controller 60 may send an alert to an operator or directly reducethe power consumption of the electrolyser 12, for example by way of asignal sent to a DC power supply within the electrolyser 12. Optionally,the controller 60 or the electrolyser operator may also send a signal tothe grid operator 26 indicating that the electrolyser 12 is reducing itspower consumption.

The electrolyser 12 is preferably sized relative to the typical flow ingas pipeline 34 such that the maximum hydrogen injection rate is rarelyexceeded. Alternatively, a hydrogen pipeline 38 may be used to connectthe electrolyser 12 to a distant but larger gas pipeline 34 or directlyto a reservoir 36. Any interim storage 52 may also be used to allow somehydrogen to be produced at some times at a rate exceeding the maximuminjection rate. However, the controller 60 may still need to reduce thenet rate of hydrogen production at some times. The net rate of hydrogenproduction can be reduced by reducing the actual rate of hydrogenproduction or by venting some of the hydrogen that is produced to theatmosphere. While venting hydrogen is not desirable, the hydrogenrecombines with vented or atmospheric oxygen to form water and does nomaterial harm to the environment.

Reducing the actual rate of hydrogen production typically requiresreducing the rate of electricity consumption. While reducing electricityconsumption is often acceptable, in some instances the electrolyser 12may be under a contract or dispatch order to consume a predeterminedamount of electricity, or the electrolyser 12 may be performing gridservices. In these cases, an unplanned reduction in electricityconsumption may harm the grid 14 or cause economic harm to the gridoperator 26 or electrolyser operator. In these cases, the electrolyseroperator may prefer to vent hydrogen.

To avoid unplanned reductions in electricity consumption or hydrogenventing, the controller 60, the electrolyser operator or another personor thing may forecast the maximum injection rates expected to exist overa period of time. The forecast can be based on one or more of currentflow rate in the gas pipeline 34, a current trend in the flow ratethrough the gas pipeline 34, historical data, or information receivedfrom the natural gas system operator. In cases where the electrolyser 12is under some level of control by the grid operator 26, or is completelycontrolled by the grid operator 26, the forecast may be converted intoan amount of available power consumption and provided to the gridoperator 26. Alternatively, the electrolyser operator or controller 60may refuse or modify a dispatch order or request for grid services fromthe grid operator 26 based on the forecast. In other cases, theelectrolyser operator can bid on energy contracts based on the forecast.The electrolyser operator may apply a factor of safety to the forecast,accept the possibility of hydrogen venting if the forecast is wrong, oreven commit to a power consumption dispatch or contract that isforecasted to cause a need to vent hydrogen.

Gas meter 58 records the amount of hydrogen added to the pipeline 34. Ina sale of actual hydrogen, the operator of the natural gas system 30 maypay the electrolyser operator for the hydrogen based on the meter 58readings at a price fixed by contract or regulations. The hydrogen mixeswith the natural gas and is thereafter considered to be natural gas.Optionally, a consumer gas meter 64 between a pipeline 34 and a consumer40 may include a hydrogen concentration sensor. The meter 64, or acomputer operated by the natural gas system operator, may adjust theactual gas flow rate to provide a flow rate of natural gas withouthydrogen having an equivalent heating value for billing purposes.Alternatively, the natural gas system operator may provide an adjustmentto a set of consumers 40 downstream of the electrolyser 12 based on anestimated reduction in the heating value of gas flowing through meter 64resulting from the hydrogen injected into the pipeline 34.

An actual sale of hydrogen may also be made to a gas consumer 40 bycalculating the amount of hydrogen actually consumed by a customer usinga calculated or measured flow rate and hydrogen concentrationinformation. Hydrogen can also be sold to a consumer 40 through ahydrogen pipeline 38 or by way of tanker trucks.

Alternatively, a virtual sale of hydrogen can be made to a consumer 40.The amount of gas withdrawn by various consumers 40 is recorded byconsumer gas meters 64. Some or all of the gas consumed by each consumer40 in an aggregate amount less than or equal to the amount of hydrogenproduced, as recorded by gas meter 58, is deemed to be hydrogen. Theconsumers 40 pay the electrolyser operator or the natural gas systemoperator for the deemed hydrogen. The hydrogen production and deemedconsumption may or may not balance in a given time period. If theproduction and deemed consumption are not the same, hydrogen is deemedto be stored or extracted from the natural gas system 30 during thattime period.

Optionally, a hydrogen reseller may contract to buy hydrogen from theelectrolyser operator and to sell hydrogen to consumers 40. Purchases bythe hydrogen reseller are paid to the electrolyser operator andsubtracted from the flow recorded through gas meter 58. Any remaininghydrogen may be sold to the natural gas system operator or othercustomers. A consumer 40 pays the hydrogen re-seller for some or all ofthe gas use indicated by use meter 64. The cost of any remaining gasused by the consumer 40 is paid to the natural gas system operator.

In any of these processes, a fee may also be paid to the natural gassystem operator for transmitting or storing the hydrogen. The hydrogenmay be billed as hydrogen or as an equivalent amount, by heating value,of natural gas.

The hydrogen can also be deemed to have been produced, or tagged, withattributes based on the source of the electricity used to produce it.For example, rather than being connected to a grid 14, an electrolyser12 may be connected only to a wind farm, solar facility or otherspecific generator 22. In a more likely scenario, the electrolyser 12can be connected to a transmission line 20 between a generator 22 andthe remainder of the grid 14. Assuming that there is no flow, or netflow, of electricity from the grid 14 to the transmission line 20, thehydrogen can be tagged as having been produced by the specific generator22. Even if there is a flow or net flow of electricity flowing from thegrid 14, some of the hydrogen can be tagged as having been produced bythe specific generator 22 based on its electricity generation. In caseswhere electricity contracts are sold in a market and the contractsdesignate the generator 22, a corresponding amount of hydrogen can betagged with having been produced by the designated generator 22.

The tagged hydrogen can be sold, in a real or virtual sale, underconditions considering the tag. For example, a consumer 40 may agree topay a higher price for hydrogen tagged as being produced by a generator22 operating from a renewable energy source such as wind, solar, biogasor syngas. Alternatively, carbon credits or offsets, tax credits orother economic or regulatory attributes of renewable energy may beassociated with the tagged hydrogen. Data relating to the production anduse of the tagged hydrogen is collected in a computer and used tocalculate an invoice for the sale of the tagged hydrogen or a record ofthe transfer of its other economic or regulatory attributes. When ahydrogen re-seller is involved, the re-seller may make a virtualpurchase of only the tagged hydrogen. The attributes of the taggedhydrogen are transferred to the re-seller who may then transfer thoseattributes to a customer. In this way, the re-seller may offer a virtualsale of hydrogen that has been produced only from renewable resources.This virtual sale might be satisfied with the delivery in fact ofnatural gas while the actual hydrogen displaces natural gas useelsewhere. Alternatively, the re-seller may allow the customer to claima carbon credit, tax credit, carbon offset or other benefit from havingpurchased the tagged hydrogen.

The electrolyser 12 is preferably connected to a high capacitytransmission line 20. In particular, the electrolyser 12 may beconnected to, or near, an interchange transmission line 18. Sinceinterchange transmission lines 18 are set up to allow electricity to besold to another control area through an imbalance market, the grid 14 isconfigured to be able to route excess electricity to these lines 18. Anelectrolyser 12 positioned near an interchange line 18 is in a locationsuitable for consuming excess power that the grid operator 26 wouldotherwise need to sell, often at negative pricing, in the imbalancemarket. Such an electrolyser 12 is also positioned to allow a gridoperator 26 to purchase, or approve a purchase, of incoming imbalanceenergy through an interchange line 18 without overloading internaltransmission lines 20.

The electrolyser 12 may be operated primarily to provide grid serviceswhen the price of natural gas does not make converting electricity tonatural gas profitable by itself. In this mode, hydrogen is produced andsold as a by-product of providing grid services but is not produced atother times. The electrolyser is off unless dispatched on, or unlesselectricity is offered at a very low rate during some particular periodof time. The electrolyser 12 may be contracted to operate as required bythe grid operator 26 or put under the direct control of the gridoperator 26. When providing grid services having a time scale of 5minutes or more, the grid operator 26 may communicate with anelectrolyser operator by way of dispatch orders and confirmationmessages. A dispatch order may be delivered by telephone, email ordedicated data link and specifies one or more desired rates ofconsumption during one or more future times periods. A confirmationmessage, delivered by the same or another form of communication,indicates that the electrolyser 12 can and will consume power asspecified in the dispatch order or indicates a portion of the dispatchorder that the electrolyser 12 can comply with. During the time periodof the dispatch order, information from a meter 84 (see FIG. 3)recording actual consumption by the electrolyser 12 is also sent to thegrid operator 26.

When the electrolyser 12 is providing grid services having a shortertime scale, such as frequency regulation, commands and confirmationmessages are sent directly between the grid operator 26 and thecontroller 60 of the electrolyser. These messages may be sent over adedicated communication link 28.

In some situations, the market price for hydrogen makes it profitablefor the default condition of the electrolyser to be full poweroperation, at least while there is sufficient demand for the hydrogen.For example, on islands such as Hawaii and Singapore, natural gas mustbe imported by tanker or produced on the island and the price of naturalgas is typically high. There is also some market for hydrogen as a fuelfor vehicles, as a heating fuel, as an industrial chemical, inwastewater treatment, in syngas upgrading, as an additive to natural gasused in vehicles, and as an additive for natural gas burned in naturalgas fired electrical generating stations. Hydrogen can also be used tomix with propane to provide a blended gas with a WAUB index similar tonatural gas. In these cases, the electrolyser 12 can provide gridservices by way of being an interruptible load or being controlled forload destruction or apparent power production.

FIG. 3 shows various electrical components of the electrolyser 12. Theelectrolyser 12 has a set of stack assemblies 80. Each stack assembly 80may in turn contain multiple electrolyser stacks. The stacks may be, forexample, alkaline or polymer electrolyte membrane (PEM) stacks. PEMstack as preferred since they are able to operate at near zero voltage,whereas alkaline stacks typically cannot operate below a significantpercentage, for example 50%, of their maximum power consumption. PEMstacks also have a greater power density and tend to be designed toproduce hydrogen at higher pressures. On the other hand, some alkalinestacks, such as those sold by Hydrogenics Corporation, are designed tobe self-pumping. This self-pumping feature can be useful when providinggrid services since it avoids the need to frequently alter the speed ofa water pump or flow control valve.

Electrical power is provided to the electrolyser 12 from the grid 14from a transmission line 18, 20 through an AC-AC step down transformer82. The step down transformer 82 reduces the voltage in the transmissionline 18, 20 to the standard step-down voltage in the control area 16,for example 120 V or 220 V. Total power consumption is tracked by aprimary electrical meter 84. AC electricity is made available to thevarious components within the electrolyser 12 through a main bus 86.

Each stack assembly 80 is separately connected to the main bus 86through an associated DC power supply 90 and stack sub-meter 88. The DCpower supply 90 should preferably have a wide voltage range, a largecurrent capacity, and a durable variable output mechanism. One suitablepower supply is a Thyrobox H2™ power supply made by AEG Power Solutions.These power supplies have a DC output of between 1V DC and 400 V DC atup to 15,000 Amps and have Thyristor based variable output mechanisms.The Thyristor mechanism is a solid state device, which avoids movingparts that might otherwise wear out with rapid power changes continuingfor long periods of time. Multiple power supplies may be provided toeach stack assembly 80 if required to provide adequate power.

The electrolyser 12 also has a balance of plant 94, parts of which willbe described further in relation to FIG. 4. Power for the balance ofplant 94 is provided from the main bus 86 through a balance of plant submeter 92.

When operating to provide grid services, for example frequencyregulation, the master controller 60 attempts to operate the DC powersupplies 90 such that the electrolyser 12 consumes a specified amount ofpower. The amount of power may be specified by an operator considering adispatch or market offering through communications link 28, or by directcontrol of the grid operator 26 through communications link 28. Thecontroller 60 may reduce the power consumed by the DC power supplies 90by the amount of power required by the balance of plant 94. This amountof power may be estimated, for example as a percentage of the totalspecified power consumption, or determined by polling the balance ofplant meter 92. The remaining power to be consumed is divided betweenthe DC power supplies 90. Readings from stack sub-meters 88 may be usedin inner control or feedback loops to adjust the requested output fromeach power supply 90 such that the actual power consumed, as determinedby a stack sub-meter 88, matches the intended portion of the specifiedpower consumption. These inner loops may also operate in sub-controllersconnected to each DC power supply 90 and its associated stack sub-meter88. Optionally, readings from the primary meter 84 may be used in anouter control or feedback loop to adjust control signals such that thetotal power consumed by the electrolyser 12, as determined by primarymeter 84, matches the specified amount. In frequency regulation, forexample, the grid operator 26 may send a series dispatches, eachdispatch indicating the required power consumption for a specifiedperiod of time. The controller 60 attempts to match the dispatches withthe timing and amount of actual power consumption as closely as possibleor at least within a range of tolerance, for example within 10% of thespecified power consumption for at least 90% of the applicable timeperiod. An acceptable tolerance for following the dispatches may bespecified by the grid operator 26 directly or by way of a penalty foroperation outside of the stated tolerance.

Providing multiple stack assemblies 80 with associated power supplies 90allows multiple stack assemblies 80 to be operated at different powerconsumptions at the same time. The controller 60 may be programmed toimplement a number of modes of operation. The controller 60 selectsbetween modes of operation at the request or control of an operator, oras a programmed response to a specified grid service commitment or toforecasted or contracted future power consumption or price.

For example, the electrolyser 12 may provide grid stabilizing services,for example primary or secondary frequency regulation services, in whichthe specified power consumption is expected to vary over a time periodof 5 cycles to 5 or 10 minutes, or over longer periods of time but in anunpredictable manner. For example, under a primary frequency regulationservices contract, the specified power consumption may vary every 4seconds, which is the time between automatic generation control (AGC)signals in some North American grids 14. Under a secondary frequencyregulation services contract, the specified power consumption may varyevery 5 to 10 minutes. In these cases, the controller 60 operates all ofthe stack assemblies 80 at about the same power level. In this way, thetotal size of a power fluctuation is spread out over the maximum numberof stack assemblies 80 to provide the minimum rate of change in eachstack assembly 80. Any excess stack assemblies 80 may be shut down orput in standby. For example, for a 1 MW electrolyser 12 having fivestack assemblies 80, a contract to provide 0.5 MW of primary frequencyregulation is met by operating three stack assemblies 80 at about thesame power level as required to satisfy the contract. The two remainingstack assemblies 80 as shut down or put on standby. However, these tworemaining stack assemblies may be activated if an opportunity arises topurchase electricity at a price low enough to profit from producingadditional hydrogen.

When providing secondary or tertiary frequency regulation services, thespecified power consumption is expected to vary over a time period of 5to 30 minutes, for example 5 to 10 minutes for secondary regulation and15 to 30 minutes for tertiary regulation. The maximum size of thevariation over a period of time may also be predictable. In this case,and in the case of other services with infrequent or predictable changesin specified power consumption, the controller 60 determines a number ofstack assemblies 80 required to produce a nominal base line consumption,being the minimum consumption expected over a frequency regulationperiod, which can be some or all of the time covered by a grid servicescontract, and operates these stack assemblies 80 at full power. Thecontroller also predicts a second number of stack assemblies 80sufficient to provide additional consumption expected during thefrequency regulation period, and operates these stack assemblies 80 withvariable power consumption. In this way, the effect of oscillations orerrors in the control loops is reduced. Further, operation at a steadystate typically causes less wear on the stack assemblies 80. Anyremaining stack assemblies 80 are shut down or put on standby. However,these two remaining stack assemblies may be activated if an opportunityarises to purchase electricity at a price low enough to profit fromproducing additional hydrogen.

For example, a 1 MW electrolyser 12 may have five stack assemblies 80.The electrolyser 12 can provide up to 0.2 MW of active frequencyregulation from one stack assembly 80 while also consuming between 0 and0.8 MW of base line consumption in the four remaining stack assemblies80. While consuming 1 MW, all stack assemblies 80 are in operation. Ifthe required base line consumption is reduced to 0.6 MW, then one stackassembly 80 can be shut down or put into a standby mode, or operated toprovide active frequency regulation. When providing base lineconsumption, it is preferable to have shut down or put into standby asmany stack assemblies 80 as possible rather than spreading the base lineconsumption over all stack assemblies 80 not needed for active frequencyregulation. However, if the electrolyser 12 is required to provide 1 MWof frequency regulation with no base line consumption (ie. the specifiedpower output is expected to fluctuate by 0.8 MW rapidly or in a mannerthat is difficult to predict), then all stack assemblies 80 operate atsimilar fluctuating power levels. As will be described further inrelation to FIG. 4, despite the variable electricity consumption andhydrogen production, the hydrogen is produced at an essentially constantpressure.

In another mode of operation, the electrolyser 12 is operated to providea hydrogen production or energy arbitrage mode. At any time whenelectricity is available at a price at which the production of hydrogenproduces a profit, then all stack assemblies 80 operate at full power.When the price of electricity is too high to profit from the sale ofhydrogen, then all stack assemblies 80 are shut down or put in standbymode. Optionally, a potential profit from producing hydrogen can becompared to profit from providing a grid service. The expected profitfrom providing grid services can be increased by an estimated value ofthe hydrogen that will be produced while providing the grid service.When the value of grid services is higher, a sufficient portion of theelectrolyser capacity is allocated to providing the grid service. Anyremaining stack assemblies 80 can be operated in hydrogen production orarbitrage mode. Optionally, when one or more of the stack assemblies 80are operating in a hydrogen production or energy arbitrage mode, thecontroller 60 may also be programmed to start and stop the one or morestack assemblies 80 such that the one or more stack assemblies 80 are onwhile electricity is available at or below a specified price.

In an automatic grid service mode, the controller 60 may be linked to agrid voltage meter 96 or other grid condition sensor. If the gridvoltage is rising or high, the controller 60 increases power consumptionuntil the maximum power consumption is reached or the grid voltagestabilizes within a target range. Conversely, if the grid voltage isdropping or low, the controller 60 decreases power consumption until thegrid voltage stabilizes within the target range.

FIG. 4 shows various elements of the balance of plant 94 of theelectrolyser 12. Each stack assembly 80 has a thermostat 90 located inthe stack assembly 80 or in an oxygen outlet 102. The thermostat 90 isconnected to a flow control valve 100 in a water input line 104 leadingto the stack assembly 80. A stack temperature controller 106, or anothercontroller such as the master controller 60, modulates the flow controlvalve 100 to maintain the temperature of the stack assembly within aspecified range. The flow of water is provided from a set of parallelpumps 108. The number of pumps 108 is one more than the number requiredto provide the maximum design water flow such that a pump 108 may beremoved for servicing. By using the flow control valves 100 to controlthe temperature of individual stack assemblies 80, fluctuations in theoperating speed of the pumps 108 are reduced and a single pump set canbe used for multiple stack assemblies 80. Water flow through a stackassembly 80 also shuts down and restarts automatically as the stackassembly 80 transitions between off or standby and operating modes.

Excess cooling water is produced with the oxygen and travels from theoxygen outlet 102 to an oxygen separator 110. The pressure within theoxygen separator 110 is maintained at a generally constant level by anoxygen regulator 112. The separated water 114 returns to the pumps 108through a cooling device, such as a radiator 116 and fan 118. The speedof the fan 118 is controlled to produce a desired temperature in thewater reaching the pumps 108. A recirculation valve 120 is modulated inresponse to the current drawn by pumps 108 to allow water recirculationin the event that closing flow control valves 100 are stressing thepumps 108. While the speed of the pumps 108, or the number of pumps 108operating, may also be reduced, repeatedly varying the speed of thepumps 108 causes them to wear rapidly.

Hydrogen is produced from a hydrogen outlet 122 of each stack assemblyand travels to a hydrogen separator 124. Separated water 126 flows to amake up water tank 128. Make up water tank 128 also receives deionizedmake up water 130 when required. A pump 108 pumps make up water into thewater recirculation circuit when the water level in the oxygen separator110 drops to a specified minimum.

The pressure in the hydrogen separator 124 is determined by a hydrogenregulator 132. Produced hydrogen is collected in the interim storagetank 52. The pressure in the interim storage tank 52 is always less thanthe pressure in the hydrogen separator 124. However, referring back toFIG. 2, a compressor 54 is operated such that pressure in the interimstorage tank is not drawn down to less than a specified part, forexample 80%, of the gauge pressure in the hydrogen separator 124 toreduce losses in pressure energy. Oxygen leaving the electrolyser 12 maypass through a pressure recovery turbine or other device to recover itspressure energy.

The seals in the stack assemblies 80 may wear out due to the product ofthe pressure applied against them over time, and due to the fluctuationsin the pressure applied against them. When the electrolyser 12 is usedto provide grid services, the power applied to the stack fluctuatesfrequently and it is beneficial to avoid corresponding fluctuations inpressure.

Pressure against the seals is kept more generally constant by ventingmultiple stack assemblies 80 to common gas separation vessels 124, 110.Particularly when one or more of the stack assemblies 80 are producinghydrogen, the pressure regulators 112, 132 are able to provide stablepressures in the gas separation vessels 124, 110 while permitting flowthrough the stack assemblies 80. Variations in the gas produced by onestack assembly 80 are dampened by the size of the gas separation vessels124, 110. The system as a whole therefore requires fewer pressureregulating devices, and the pressure regulating devices can have reducedmovement, relative to a plant having gas separation vessels for eachstack assembly. Pressure following between the oxygen regulator 112 andthe hydrogen regulator 132 maintains almost equal pressures on bothsides of the membranes of the stack assemblies 80.

The gas outlets of the stacks are located on the top of the stacks andwater flows upwards through the stacks. When a stack is powered down,its residual gas bubbles flow upwards to the gas separators 110, 124.Water drains down from the gas separators 110, 124 into the stackassemblies 80. The water in the stacks prevents residual hydrogen andoxygen from reacting and either degrading the materials, for example thecatalysts, in the stack or converting the stack into a fuel cell. Thewater also preserves the pressure in the stack to reduce pressurefluctuations. In this way, power can be reduced to a stack withoutrequiring other changes in the balance of plant.

Referring to FIG. 5, a consumer 40 may extract gas from the pipeline 34through a hydrogen enriching extraction device 134. Multiple meters 64allow the amount of gas withdrawn from the pipeline 34, returned to thepipeline 34, and delivered to the consumer 40 to be measured. Isolationvalves 136 optionally allow the extraction device 134 to be operated ina batch mode. A compressor 138 allows hydrogen depleted gas to bereturned to the pipeline 34.

The consumer 40 may be, for example, a natural gas fuelling station or anatural gas fired electrical generating station. A variable lowconcentration, for example up to about 5%, of hydrogen can be added tonatural gas without materially changing the operation of typical gasfired appliances such as a household furnace. Engines, however, can bemore sensitive to changes in the composition of their fuel. Natural gasengines include gas fired turbines used to generate electricity andinternal combustion engines in vehicles. However, a moderateconcentration of hydrogen, up to about 15%, can be beneficial to theoperation of a natural gas engine and can reduce emissions of carbondioxide and pollutants.

The control system of a natural gas engine may sense the hydrogenconcentration of the fuel, the operation of the engine in response tothe hydrogen content of the fuel, or both, and react accordingly.Alternatively, at a vehicle fuelling station or in a supply system for anatural gas fired turbine, natural gas may be provided enriched withhydrogen in one or more grades of generally constant concentrationsgreater than what is present in the pipeline 34.

For example, a 5% or 10% hydrogen enriched natural gas product can beproduced by measuring the concentration of hydrogen in natural gaswithdrawn from the pipeline 34 and adding a required amount of hydrogento reach the desired concentration. Optionally, the desired amount ofhydrogen can be determined by monitoring a hydrogen concentration sensorin addition to, or instead of, pre-calculating the required amount. Inthis case, measuring the concentration of hydrogen in natural gaswithdrawn from the pipeline 34 can be omitted. The additional hydrogencan be provided by hydrogen pipeline 38, tanker truck, on siteelectrolysis, or on site conversion of natural gas, for example by steamreformation. The electrolyser 12 may also be co-located with a fuellingstation or electrical generating station, or connected by a hydrogenpipeline 38, to provide hydrogen for natural gas enriching directly.

Alternatively, a mixture having an increased or decreased concentrationof hydrogen can be extracted from the pipeline 34 through the extractiondevice 134. If the extracted mixture has more than the desired hydrogenconcentration, the hydrogen can be diluted to the target concentrationwith non-enriched gas taken from the pipeline 34.

The extraction device 134 can remove hydrogen enriched gas for exampleby a hydrogen selective membrane such as a membrane made of palladium ora palladium silver alloy. The pipeline gas can also be compressed toseparate the gasses by liquefying only one of them. Hydrogen enrichednatural gas may also be withdrawn by an absorbent in the extractiondevice 134. The absorbent may be a metal hydride forming metal such aslanthium nickel or iron-titanium or carbon nanotubes. In this case, theextraction device 134 is a tank filed with the absorbent. By operatingvalves 136 and compressor 138, pipeline gas is passed through the tankat pipeline pressure, which causes hydrogen to be captured as a metalhydride. The tank is then isolated from the pipeline 34 and vented tothe consumer 40. When pressure in the extraction device 134 is reduced,adsorbed hydrogen is also released creating a hydrogen enriched naturalgas.

The amount of hydrogen removed by the fuelling station or electricitygenerating plant may be monitored for billing separately from thenatural gas. The hydrogen, or hydrogen enriched natural gas, may carry adifferent price, a carbon credit or renewable energy benefit. Thefuelling station may pass on this benefit to the customer, or offer thecustomer the option of purchasing a lower emissions or partiallyrenewable fuel.

In an energy system 10 with multiple electrolysers 12, the operation oftwo or more electrolysers 12 may be controlled together as a fleet. Forexample, if it is desirable for the grid 14 to have excess electricityconsumed, a fleet controller may operate an electrolyser 12 that issubject to fewer electrical transmission restraints, is subject to fewerrestraints on the amount of hydrogen that can be injected to a pipeline,or is better able to serve a market for a higher value hydrogen use.

Optionally, hydrogen may be used or injected into a pipeline indirectly.In particular, the hydrogen may be input into a process to createmethane. The methane may then be injected into a gas pipeline orsupplied to a vehicle fuelling station or electrical generating station.Hydrogen can be converted into methane by, for example, a Sabatierprocess. In addition to hydrogen, the Sabatier process requires a sourceof carbon dioxide. The carbon dioxide can be extracted from the exhaustfrom an engine or furnace, from biogas, or from another carbonsequestration processes. Alternatively, hydrogen can be converted intomethane by adding the hydrogen to an anaerobic digester, for example adigester being used to produce methane from biomass. The hydrogen iscombined with carbon dioxide in the digester to create methane andincrease the methane output of the digester. Alternatively, the hydrogenmay be combined outside of the digester with carbon dioxide separatesfrom biogas produced by an anaerobic digester.

Converting the hydrogen to methane consumes about 20% of the potentialenergy of the hydrogen. However, some of this loss may be recovered aswaste heat. Further, methane may be injected in unlimited amounts into anatural gas pipeline. Methane also has about three times the energydensity of hydrogen and is the primary fuel of existing natural gasfuelled vehicles. Accordingly, in some cases the conversion to methanemay be desirable. In particular, if the cost of producing the hydrogenis at least partially covered by providing grid services, if theconsumption of carbon dioxide in the methanation process provides abenefit such as a carbon credit, or if the hydrogen or the electricityused to produce the hydrogen would otherwise have been wasted, thenproducing methane can be a viable use for the hydrogen.

The invention claimed is:
 1. A process comprising the steps of, a)providing an electrolyser in communication with a controller; b)receiving a series of dispatch orders from a grid operator of anelectrical grid indicating a specified power consumption for a period oftime, the dispatch orders occurring at least once every 30 minutes; c)controlling the electrolyser according to the dispatch orders such thatthe electrolyser consumes the specified power consumption for the periodof time indicated in the dispatch orders; and, d) discharging hydrogenproduced during step c) to a natural gas system.
 2. The process of claim1 wherein the dispatch orders occur at least once every 10 minutes. 3.The process of claim 1 wherein step c) comprises operating theelectrolyser according to the dispatch orders within a specifiedtolerance.
 4. The process of claim 1 wherein step d) comprisesconverting the hydrogen into methane before discharging it into thenatural gas system.
 5. The process of claim 1 wherein the electrolyserhas an electricity meter, and wherein in step c) the electrolyser iscontrolled to operate a DC power supply so as to consume the power at apre-determined rate.
 6. The process of claim 1 wherein in step c) theelectrolyser is controlled according to the dispatch orders as part ofan aggregation of i) multiple electrolysers or ii) at least oneelectrolyser and at least one other variable load.
 7. The process ofclaim 1 wherein the electrolyser has multiple stack assemblies eachhaving a separate power supply and in step c) the controller controlsthe multiple stack assemblies to operate at different rates of powerconsumption at the same time.
 8. The process of claim 1 furthercomprising steps of, a) receiving, at the controller, data related to amaximum amount of hydrogen that may be injected into the natural gassystem; and, b) performing, via the controller, one or more of i)controlling the electrolyser to consume no more than an amount ofelectricity that will produce the maximum amount of hydrogen, (ii)venting excess hydrogen and (iii) sending a signal to the grid operatorindicating a corresponding maximum amount of hydrogen that can beproduced.
 9. The process of claim 8 wherein the data comprises the flowrate of natural gas in the natural gas system.
 10. The process of claim1 wherein the electrolyser has multiple stack assemblies that ventupwards to shared gas separators.
 11. The process of claim 1 comprisingsteps of, a) withdrawing natural gas from the natural gas system; b)measuring the concentration of hydrogen in natural gas withdrawn fromthe natural gas system; and, c) producing a gas mixture with a specifiedhydrogen concentration by i) adding hydrogen to the natural gas or ii)blending the natural gas with a hydrogen enriched mixture of hydrogenand natural gas withdrawn from the natural gas system.
 12. Anelectrolyser having a controller, an electricity meter, and multiplestack assemblies each having a separate DC power supply, wherein thecontroller is adapted to operate the DC power supplies so as to causethe multiple stack assemblies to consume power at different rates at thesame time to collectively consume electricity at a pre-determined rate.